Additional Factors to Consider to Minimize Re-suspension

Additional Factors to Consider to Minimize Re-suspension

• 
  Hydraulic Dredging Percent Solids and Production Rates - The potential for re-suspension and leaving contaminated residue is reduced by reducing the percent solids in the dredge slurry and increasing the production rates. However, this can significantly increase the volume of dredge water that must be treated and handled. The dredging operation should be designed to minimize re-suspension, while taking cost of water treatment into consideration.
  
• 
  Cutter Head Design and Swing Rate – The type of cutter head affects the amount of re-suspension that occurs, and the faster the swing rate of the head the more sediment will be suspended in the water column. However, slow swing rates will also prolong the dredging activity. Again, the dredging operation should be designed to minimize re-suspension as much as possible, while taking the cost of dredging time into consideration.
  

Possible Methods for Residue Management to Minimize Re-suspension

• 
  Multiple Pass Dredging – Multiple pass dredging can remove contaminant residue left by deposition of sediments suspended in the initial dredging operation. The cost of multiple pass dredging should be balanced against the additional contaminant reduction achieved.
  
• 
  Over-Dredge Allowances – Setting a specific over-dredge allowance for a dredge contractor may minimize residual contamination that is a result of missed target material. By doing this, a dredge contractor may be more likely to achieve a specified “neat line” elevation without being penalized for minimal over dredging.
  
• 
  Environmental-specific Mechanical Dredge Buckets - These types of dredge buckets have gasket sealed joints and hydraulic pressure release valves that are specifically designed to minimize sediment suspension during mechanical dredging. Bucket rinse tanks are also used to remove sediment from the bucket between each scoop.
  
• 
  Specially Designed Cutter Heads and Shrouds - This type of cutter head dredge may control re-suspension of sediments and reduce residual contamination.
  
• 
  Silt Curtains - Depending on the site conditions, silt curtains may be appropriate to control sediment movement. Silt curtains are most effective if they can be placed once, since placement and removal of silt curtains can cause sediment re- suspension. Generally, silt curtains work best if they are not in contact with the bottom, which can cause sediment suspension in tidal and faster moving river locations. For more on silt curtains see Silt Curtains as a Dredging Project Management Practice (Ref. V.1a.10).
  
• 
  Post Dredge Cover or “Dilution Cap” - This can be utilized where residue management efforts are not expected to meet site remediation goals, a post-dredge cover or “dilution cap” can provide the means to reduce potential exposures to the residual contamination.
  

Resources

• 
  Innovations in Dredging Technology: Equipment, Operations, and Management – April 2000 (Ref. V.1a.5)
  
• 
  Innovations in Dredging Technology: Equipment, Operations, and Management – February 2000 (Ref. V.1a.6)
  

Section V.1b Excavation

Sediments can be removed by excavation from the upland shoreline or by isolating an area to make it dry with dams or sheetpile walls. Besides excavation equipment, sediment transport equipment (typically trucks) will be required. Sometimes in river or stream excavations, the river is blocked upstream and downstream. This will generally require the water to be diverted around the area being excavated.

Section V.1c Capping

Capping involves covering sediments with the proper type and amount of material to prevent the movement of contamination. Typically, a cap should be placed in a location where the cap will not move (i.e. low flow areas) nor interfere with access to utilities or structures. Cap design and construction will need to include a variety of considerations. The physical properties of the sediment in the area to be capped needs to be determined prior to cap placement. The proper placement of the cap material is critical to the success of the remediation. Generally, cap material is placed slowly over the contaminated area to prevent contamination migration. The more unstable the contaminated area, the slower the initial cap placement will need to be. Sometimes caps are placed in layers (or lifts) and each layer is allowed to consolidate. The cap material has to be the correct size and density range to prevent the cap and contaminated material from moving. Sometimes heavier material is placed on the top to protect the cap. The cap has to be correct thickness to ensure the contaminated sediments are isolated and to prevent the contaminants from migrating through or around the cap.

Unless the contaminated area is deep, capping is generally not done near navigation areas. Any capped area should be monitored to determine that there is no contamination migration through or around the edge of the cap.

• 
  Changes to Bottom Typography - Capping will change the bottom typography of the surface. This is most important in navigational channels and at shoreline, near shore, or shallow areas. The depth and the velocity of the water can affect the type of plant and animal communities. Capping in shallow areas can cause significant changes to the plant and animal habitats. Depending on the site specific locations, replacement may not be necessary because plant and animals may return naturally. Consult with appropriate regulatory agency for mitigation requirements. Marine charts may need to be updated depending on the extent of impact to navigational area.
  

• 
  Potential Biologically Active Zone (BAZ) of Sediments - The BAZ is the upper layer of the sediment in which both plants and benthic organisms are active.  It is necessary to understand the site specific BAZ to determine the thickness of post remediation sediments that must meet cleanup goals to remain protective for human and ecological receptors. For a capping remedy, the lower portion of the cap material is often referred to as the Isolation Zone (IZ).  The IZ is the portion of the cap that prevents the underlying contamination from moving by advection or diffusion into the BAZ above the acceptable protective level.  The BAZ-portion of the cap becomes the new ecological substrate for benthic invertebrates and rooted aquatic plants.  Aquatic plants can uptake contaminants through their root systems causing direct toxicity or transferring them up the food chain.  Their root systems and root channels can also cause preferential groundwater flux. Similarly, benthic organisms can burrow into contaminants, creating pathways for contaminant migration or transfer up the food chain. Therefore, the BAZ and IZ must be properly scaled to prevent site flora and fauna from encountering site contaminants.
  
• 
  Ice and Freeze/Thaw Effects on Sediments - Ice movement and freeze/thaw events can have a substantial affect on soft sediments and cap materials.  Ice push and ice heave due to wind driven movement, thermal expansion, or currents can affect contaminated sediment distribution and cause substantial damage to in-situ remedial caps.  Also, the freeze/thaw process can cause ice intrusion, ice wedges, matrix alterations, and permeability/hydraulic conductivity changes.
  
• 
  Factors to consider when evaluating potential for ice effects include:
  

• 
  Wind fetch distances,
  
• 
  Basin characteristics,
  
• 
  Snow cover (thermal insulation),
  
• 
  Temperature fluctuation, and
  
• 
  Existing evidence of ice damage or ice push events
  

Resources

• 
  U.S. EPA Guidance for In-Situ Subaqueous Capping of Contaminated Sediments (Ref. V.Ic.1)
  
• 
  Equipment and Placement Techniques for Subaqueous Capping (Ref. V.Ic.2)
  
• 
  Guidance for Subaqueous Dredged Material Capping (Ref. V.Ic.3)
  
• 
  Hazardous Substance Research Center South & Southwest In-situ Capping Primer (Ref. V.Ic.4)
  
• 
  Subaqueous Capping and Natural Recovery: Understanding the Hydrogeology setting at Contaminated Sediment Sites (Ref. V.Ic.5)
  
• 
  Subaqueous Cap Design: Selection of Bioturbation Profiles, Depths, and Process Rates (Ref. V.Ic.6)
  

Section V.1d In-Situ Remediation

In-situ remediation involves an active remedy in-place. This type of remedy is best suited for contaminants that can be broken down chemically or biologically (see Section V.1e), absorbed, or encapsulated. In general, these types of remedies are not considered fully developed. However, there have been several pilots study tests of these types of remedies.

Section V.1e Bioremediation
Depending on the type of contamination (i.e., materials that will degrade within a reasonable time frame), bioremediation can be utilized to beak down the contaminants. Typically, additional nutrients or specific microbes are required to perform bioremediation. Bioremediation has been more successful in certain soil and groundwater remediation scenarios than sediment remediation. Bioremediation will not reduce metal contamination. Because of the extra cost of moving sediments and biological treatment systems, bioremediation is generally done in-situ. A monitoring program may include sediment sampling, and monitoring the contamination levels and any breakdown products resulting from the bioremediation.
Issues

• 
  Selection of Bacteria - The selection of the correct type of bacteria is critical to successful bioremediation. The type(s) of contaminant(s) will dictate the type of bacteria. Because the final breakdown products must be less toxic than the original contaminants, the mechanism of the breakdown process must be understood. It is also important that all intermediate breakdown products not be significantly toxic or more mobile than the original contaminant(s). The bacteria selected must not be toxic to local plants and animals, or humans. A laboratory study should be conducted to determine the breakdown mechanism and final breakdown products. A pilot study at the site should be done before attempting large scale bioremediation because field conditions can be significantly different than laboratory conditions.
  

Section V.1f Natural Attenuation
Natural attenuation is also known as Monitored Natural Attenuation (MNA). EPA describes the natural attenuation processes to “…include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in-situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants.” from Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage tank Sites (Ref. V.1f.1). MNA requires modeling and monitoring to prove its effectiveness as a remedy.

Directory: Files
Files -> Answer True False 2 points Question 2
Files -> Integer programming and game theory
Files -> 4 Integer Programming
Files -> Bpa vehicle Window Repair Scenario #1 task: Procure vehicle window relacement. Objective
Files -> Pop Warner History
Files -> North Carolina Inclusion Initiative Mapping Where Children with ieps are Being Served Purpose
Files -> Fall 2013 Spring 2014 Program Data: Standard 1 Exhibit 4d
Files -> Hanban – asia society confucius classrooms network 2010 request for proposal
Files -> Northern England’s set-jetting locations

Download 0.75 Mb.

Share with your friends: